This invention relates to tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxymethyl]methane [hereinafter called compound (1)] in the grained alpha-form crystals having improved coloration, and a process for the production thereof.
Compound (1) is disclosed e.g. in U.S. Pat. No. 3,644,482 and has been accepted as excellent anti-deterioration agent against heat and oxidation for organic material such as synthetic polymers like polyolefines, polyesters, polystyrenes, polyurethanes, polyvinyl chlorides, etc. With a recent wide use of these synthetic polymers as durable consumer goods such as automobiles and electric appliances, demand for higher qualities of additives for plastics is growing. Especially, the colorlessness of such stabilizing agents is essential since coloration of final plastic products is degraded by the use of colored agents. And also, demand for powder-free additives is increasing from the industrial viewpoint of handleability, transportability, flowability, etc. The compound (1) available on the market is, however, a little colored or in the form of powder or aggregate of fine crystals having a low mechanical strength to be crushed during handling, storage and/or transportation, resulting in low bulk density, inferior fluidity and easy scattering.
According to U.S. Pat. No. 3,644,482, Compound (1) is prepared by transesterifying low-alkyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate [hereinafter called compound (2)]with pentaerythritol in the presence of alkali metal hydride as catalyst and absence of reaction solvent or presence of dimethylsulfoxide, followed by the recrystallization from isopropanol or n-hexane. The compound (1) obtained by the above method is, as described in U.S. Pat. No. 4,405,807 colored yellow and can not be supplied to the market.
Japanese Pat. No. 13018/60 (corresponding to U.S. Pat. No. 4,405,807) states that compound (1) is obtained by reacting compound (2) with pentaerythritol in the presence of alkali metal methoxide and dimethylformamide with removal of the formed low alcohol under reduced pressure, adding isopropanol to isolate a molecular adduct compound of the compound (1) and isopropanol, and subsequently recrystallizing it from ethanol and/or methanol. The patent also refers to dimethyl sulfoxide, dimethyl formamide, tetrahydrofuran, dioxane, diglyme, dimethyl acetamide, hexamethyl phosphoramide, 1,2-dimethoxyethane, acetonitrile, propionitrile and t-butyl alcohol, which are all polar solvents, as examples of usable reaction solvents; and refers to alkali or alkaline earth metal halides, lower alcoxides of alkali metals, and metallic sodium and metallic potassium, which are derivatives of either alkali metals or alkaline earth metals, as examples of usable catalysts.
U.S. Pat. No. 4,739,102 discloses a process for the production of compound (1) in the beta-form of crystals, where the compound (1) is prepared by reacting compound (2) with pentaerythritol with removal of the formed low alcohol under reduced pressure, in the presence of alkali metal amide or other alkali or alkaline earth metal derivatives as catalyst and a small amount of aromatic hydrocarbon solvent like tetraline or other polar solvents, distilling off the solvent under reduced pressure, and then recrystallizing the product from a mixture of methanol and/or ethanol and water.
The above proposed processes for the manufacture of compound (1) have been developed only by modifications of recrystallization procedures, due to the difficulty in completion of the transesterification reaction without formation of colored by-products which are difficult to be removed by the recrystallization from aliphatic hydrocarbons. By reason of decoloration, the compound (1) has been recrystallized from low alcohol such as methanol, ethanol or a mixture of it and water. The use of such polar solvent as recrystallization solvent is effective on removal of colored matter from the compound (1). The crystals obtained by the recrystallization are, however, in the form of powder or aggregate of fine crystals with unfavorable characters such as low fluidity and low mechanical strength.
In the conventional transesterification processes for the production of compound (1), a polar solvent is used as reaction solvent. As the starting material, pentaerythritol, has an exceedingly high polarity and high melting point of 261.degree. to 262.degree. C., the use of polar solvents is effective to dissolve the said starting material. Such solvents, however, have not brought about a positive effect on lowering the formation of colored by-products, but rather made it difficult to complete the transesterification reaction, resulting in prolonged reaction time or less conversion.
Compound (1) has been known, as disclosed in U.S. Pat. No. 4,405,807, to have 5 crystal forms, i.e. alpha-form, beta-form, gamma-form, delta-form and lambda-form. The compound (1) widely available on the market has a crystal structure of beta-form having the melting point of 113.degree. to 115.degree. C. However, the said form crystals are, as described in U.S. Pat. No. 4,739,102, substantially an aggregate of fine crystals, which leads to be readily crushed and reform agglomerates.
According to U.S. Pat. No. 4,405,807 and 4,739,102, alpha-form crystals of compound (1) are obtained by the recrystallization from hexane or heptane; and the said crystals are in the shape of granules or grains having the melting point of 120.degree. to 125.degree. C., which is higher than that of beta-form. Although physical properties of alphaform crystals are superior to those of beta-form, the former crystals obtained by the conventional transesterification methods are, as described in U.S. Pat. No. 4,405,807, colored yellow and have not been supplied to the market.
The purpose of the present invention is to provide a new transesterification process in which compound (1) is manufactured without coloration. The new process found has surprisingly decreased the formation of colored by-products, by the use of aliphatic hydrocarbons as reaction solvent having the high boiling point of 100.degree. to 300.degree. C., preferably 155.degree. to 205.degree. C. Such improved process of transesterification have made it possible to obtain compound (1) in the form of colorless grained alpha-form crystals by simple crystallization from aliphatic hydrocarbons; and additionally, very little formation of by-products have made the grain size of crystals to be larger. And also, the use of organo-tin compounds as catalyst together with the said aliphatic hydrocarbon solvent increases the colorlessness of crystals. Furthermore, a much whiter compound (1) is obtainable by the direct crystallization from the reaction mixture without the step of reduced-pressure stripping. This also simplifys the manufacturing process of compound (1), producing an industrially beneficial effect.